1. Field of the Invention
The present invention relates to a field of light emitting device using light emitting element, more particularly, a light emitting device for multi-color display.
2. Description of the Related Art
Recently, research and development on the image display devices is vigorously conducted. As a display, liquid crystal displays using liquid crystal elements for displaying images are widely used today as displays on mobile telephones and personal computers, with making best use of their advantages such as high-quality image, and thin and light body.
At the same time, the development of light emitting devices using light emitting elements is also underway. A light emitting device of this type has many advantages such as quick response, capacity for displaying moving pictures, and wide field of view, in addition to the above advantages of existing liquid crystal displays. Therefore, the light emitting device using light emitting elements attracts attention as a next generation flat panel display for small mobile devices which are capable of providing moving picture contents.
A light emitting element is made from various materials including organic materials, inorganic materials, thin-film materials, bulk materials and dispersing materials. Among them, an organic light emitting diode (OLED), mainly consisting of organic materials, is one of representative light emitting elements. The light emitting element has a structure consisting of an anode and a cathode, and a light emitting layer interposed therebetween. The light emitting layer comprises one or more materials selected from the above materials.
In these days, light emitting devices in which each pixel is divided into three sub-pixels are being developed actively. Each of the three sub-pixels corresponds to the light's three primary colors R (red), G (green) and B (blue), respectively. The light emitting device provides multi-color display by displaying each sub-pixel corresponding to each color with gradation. Examples of methods for multi-color display include a method in which three light emitting elements are made from three light emitting materials corresponding to R, G and B, respectively, a method in which light emitting elements emitting white color are combined with color filters for R, G and B, respectively, and a method in which light emitting elements emitting any color are combined with color conversion materials (such as fluorescent materials).
In the light emitting device, multi-color can be displayed by using so-called additive color mixing method in which a variety of colors are generated by combining R, G and/or B. This technique utilizes a fact that human eyes have sensors sensitive to the wavelength of a light, and recognize colors by dividing the wavelength of the light incident on the eyes.
Next, the above mentioned additive color mixing will be discussed with reference to the FIGS. 8A and 8B. The FIG. 8A shows a graph in which brightness is plotted on the vertical axis and the light wavelength is plotted on the horizontal axis. As can be seen in FIG. 8A, a visible light can be divided into three regions according to the length of its wavelength. The long wavelength represents red, medium wavelength represents green and short wavelength represents blue. Also as can be seen in FIG. 8B, yellow, magenta and cyan are generated by combining the three light's primary colors. When nearly equal amount of red light, green light and blue light enter the eye, the eye recognizes the light as white color. Thus, by adjusting brightness (balance) of the three primary colors (red, green, blue), a variety of colors can be reproduced.
As for driving methods of a light emitting device, the analogue gradation method and the digital gradation method are currently in use. In the analogue gradation method, the amount of the current flowing through the light emitting element is controlled to generate gradation. In the digital gradation method, the light emitting element is driven by switching between two states, ON (almost 100% luminance) state and OFF (almost 0% luminance) state. Namely, the digital gradation method as is can display only two gradations. Therefore, methods which combines the digital gradation method with other method to display colors in multi-gradations have been proposed. Examples of such combined method for reproducing multi-gradation colors include an area gradation method and a time gradation method.
The driving methods of the light emitting devices for displaying multi-gradation image include a voltage input method and a current input method. In the voltage input method, a video signal (voltage) input into a pixel is input into the gate electrode of a driving element, which, in turn is used to control the luminance of the light emitted from the light emitting element. In the current input method, a preset signal current flows from one electrode to another electrode of the light emitting element, in order to control the luminance of the light emitted from the emitting element. Either the voltage input method or the current input method is applicable for the analog gradation method or the digital gradation method.
Different light emitting material for emitting different color necessary for the multi-color display has different current density for achieving certain luminance. For example, in the various light emitting materials for emitting one of light's three primary colors, materials for red typically have lower luminance than those for blue and green.
Furthermore, a color conversion layer of a color filter or a fluorescent filter has different transmittance for different color. Therefore, even when the light emitting elements emit light with uniform luminance, the light passing through the color conversion layer will change the luminance.
When above light emitting materials or color conversion layers such as color filters are used in the sub-pixels without modification, the lights emitted from each sub-pixel may have different luminance from each other. Also, as discussed with reference to the FIGS. 8A and 8B, white color is represented by emitting lights three primary color RGB at the same time. Therefore, if there is any difference in luminance among three colors, white color displayed on the screen may be biased to red or blue, thus, is not accurately reproduced. The luminance on the display may be uneven, or white balance may be impaired, and desirable color and image with accurate gradation cannot be reproduced.